In many integrated circuit designs, it is important to control the voltage level of the substrate relative to the operating devices within the integrated circuit. It is common to provide a substrate bias of one to two volts below the lowest voltage level provided by the voltage supply. Often this is done by a technique called charge pumping.
A charge pump is a circuit designed to pull charge on or off of a particular biased node by providing an oscillating signal to the input termianl of the charge pump. The oscillating signal causes a capacitor to be charged on one-half cycle of the signal and the charge to be forced on to the biased node during the other half cycle of the signal. This technique is very useful in complimentary metal oxide semiconductor circuits where latchup is a concern and in dynamic random access memories where precise control of the capacitance of a capacitor in the memory is necessary.
Integrated circuits are required to operate over a wide range of supply voltages and temperature conditions. Thus, the charge pumping circuitry must be robust enough to provide the required voltage level under a wide range of operating conditions. However, a conventional charge pump designed to be robust enough to deal with high-load operating conditions tends to draw a great deal more power than necessary when the circuit is operating at near optimal conditions.